1.—Technical Field
Embodiments disclosed herein relate to the field of optical sensors. More particularly, embodiments disclosed herein are related to the field of modulated sources for optical sensors to measure the chemical composition in fluids.
2.—Description of Related Art
Optical sensors typically use a light source to provide the excitation of atoms or molecules for prediction or detection of chemical constituents contained in the liquid, solid, or gases under examination. The light source is provided as part of the system or sensor used to study these materials and generally cover a portion, or portions, of the electromagnetic spectral region from the ultraviolet (UV) to the far infrared (FIR). Different types of light sources used may provide a continuum emission, line emission, and continuum plus line and quasi-continuum emission, according to the spectral distribution of the light provided. Examples of these types of sources are incandescent lamps, hollow cathode discharge lamps, xenon-mercury arc lamps, light emitting diodes (LEDs) and lasers, respectively.
Typically, continuum sources like tungsten lamps are operated at a fixed, continuous voltage, or current. In typical systems, the detected signal is derived from modulating the light beam with the use of an optical chopper or similar device. Light beam modulation improves the measurements made with these devices by moving the detected signal from a direct current (DC) domain to the alternating current (AC) domain, improving signal drift over time.
An alternative approach is to utilize modulation of the light source itself, without the use of external elements such as optical choppers or modulators. Current systems that modulate light sources typically include semiconductor light sources. In some applications, a modulated current is applied to a semiconductor light source having an emitter. The current modulation produces a temperature modulation in a gain region of the emitter, thus changing the emitted center wavelength. This method has been used to measure properties of a downhole fluid in oil exploration and extraction applications. However, light source modulation methods such as used in state-of-the-art applications include light sources providing a limited bandwidth such as lasers and semiconductor light sources, typically emitting light within a narrow wavelength band.
What is needed is an optical source and a method for using the optical source providing measurement stability and high signal-to-noise (SNR) ratio in a broad spectral band, to determine the chemical composition of fluids including hydrocarbons (e.g., crude oils, natural gas, or the like) and/or other fluids present in a downhole environment.